These studies are directed at elucidating the factors involved in the formation of specific patterns of connections in the central nervous system, and in particular the topographic representation of the retina upon the optic tectum. For most of this work the chick visual system has been used as a convenient model since much is now known concerning its development and adult morphology. Currently the effects of partial lesions of the optic cup upon the development of the retino-tectal projection are being examined, and using a recently developed autoradiographic method for tracing axonal connections we are examining the whole sequence of invasion of the optic tectum by retinal fibers and the mechanism of synapse formation between labeled retinal ganglion cell axons and tectal neurons.